The present invention relates to balun transformers, in particular it relates to balun transformers with one or two balanced ports and an unbalanced port which have a small size, low microwave losses and which most particularly are suitable for Monolithic Integrated Circuit (MMIC) applications. Particularly they should be small enough for applications at low frequencies, particularly below about 10 GHz.
A balun (balanced-to-unbalanced) transformer is a (passive) two or three-port electronic circuit with a functionality of converting an unbalanced signal (unbalanced in relation to ground) into balanced signals and vice versa of converting balanced signals into an unbalanced signal. It is generally used for conversion between balanced (symmetrical) and unbalanced (non-symmetrical) transmission lines. A signal incoming to an unbalanced port may be divided between two balanced ports providing signals which have the same amplitude but phases differing 180xc2x0 in relation to one another. Baluns may for example be used in transmitting circuits and receiving circuits of mobile communication devices, for construction of balanced amplifiers, mixers, VCOs (Voltage Controlled Oscillators), antenna systems etc.
Generally baluns are widely used in microwave devices and microwave systems. As referred to above, a balun can be used both as a two-port component and as a three-port component.
In FIG. 1 a (state of the art) two-port balun, or rather the lumped circuit equivalent of such a balun, is illustrated. The balanced port is formed between terminals T3 and T4 respectively, at which terminals the potentials are V3 and V4 respectively, of a primary coil whereas the unbalanced port is formed by terminals T1 and T2 respectively of a secondary coil. The potential at T2 of the unbalanced port is zero whereas T3 is not balanced, here having a potential denoted Vu. The unbalanced port may be connected to a microstrip, coplanar waveguide (CPW) or some other kind of unbalanced lines or components. For a two port component, the balanced port may be connected to components which have no ground plane on the back side of the substrate such as an inductor or a coplanar strip waveguide (CPS).
FIG. 2 shows a lumped circuit equivalent of a balun with a ground plane on the back of the substrate, i.e. of a three-port component. Again terminals T1 and T2 form the unbalanced ports with V2=0 and V1=Vu, T2 being connected to ground. A first balanced port, port Pb1, formed by terminals T3 at potential V3=Vb and T5 at potential V5=0 (however, T5 does not have to be grounded, it may also be a fixed voltage) whereas the second balanced port, port Pb2, is formed by T4 and T5, wherein the potential at T4 is V4=xe2x88x92Vb. The potentials at terminals T3 and T4 are 180xc2x0 out of phase with respect to the terminal T5 and T5 corresponds to the ground plane on the backside of the substrate on which the balun is provided. For one three-port configuration component, T5 should be in galvanic contact with the ground plane. The balun of FIG. 2 may also be used in a two port configuration where the balanced port is formed between terminals T3 and T4, whereas terminal T5 may be galvanically connected to the ground plane, (although this is not necessarily the case). It may also be connected to a DC source.
In the past several attempts, using different approaches, have been made to reduce the size of a balun. For microwave frequency applications it is known to realize baluns on sections of transmission lines. In xe2x80x9cMiniaturised Lumped-Distributed Balun for Modern Wireless Communication Systems, IEEE, MTT-Sxe2x80x277, pp. 1347-1350xe2x80x9d by Ojha et al, a distributed microstrip balun is suggested which incorporates lumped capacitors to reduce the balun size. However, the balun is still large, it has a length exceeding 5 mm and it is also a narrow band (0.864-0.896 GHz) balun. It is also disadvantageous that the lumped capacitors have to be wire bonded which makes the design expensive.
In xe2x80x9cA reduced Size Planar Balun Structure for Wireless Microwave and RF Applications, IEEE MMT-Sxe2x80x296, pp. 526-529xe2x80x9d by Preetham et al, another design based on coupled microstrip lines is discussed in which the effective length enhancement of transmission lines is used by employing capacitive effects. In such a design no lumped capacitors are needed but the balun will still be a large, narrow band device which further is not compatible with MMIC technology.
In xe2x80x9cModelling and design of Novel Passive MMIC Components with Three and More Conductor Levels, IEEE MTT-Sxe2x80x294, pp. 1293-1296; Improved compaction of multilayer MMIC/MCM baluns using lumped element compensation, IEE MTT-Sxe2x80x297, pp. 227-280xe2x80x9d by R. Jansen et al, baluns based on lumped inductors are proposed. Although it is possible to achieve a substantial reduction in size, which makes it possible to fabricate the baluns on semiconductor chips (MMIC) the losses would still be high if low resistivity silicon substrates were used.
Baluns may also be realized by transformers based on lumped inductors. It is for example known to realize baluns by ferrite transformers. However, the microwave losses would be high for such baluns and they are not suitable for applications in MMIC technology. Lumped inductor based transformers may be used to make small on-chip baluns in MMIC technology for low frequencies. However, for conventional silicon MMICs, the losses due to the high conductivity of the silicon used in standard manufacturing technology would be high.
Baluns based on sections of transmission lines, microstrip lines, CPW (Coplanar Waveguide) etc. generally have low microwave losses. However, they are large and that limits the possibilities of using them in MMIC applications, especially at low frequencies (less than 10 GHz). U.S. Pat. No. 5,061,910 shows a distributed balun. This balun requires a plurality of conductor elements of xcex/4 length. This means that the balun will be large. Thus, also the balun disclosed therein is not small enough and not applicable at low frequencics. Further yet it is not appropriate for MMIC implementations.
What is needed is therefore a balun transformer which is small and which has a good performance. The performance is given by the insertion losses, the reflection losses and the bandwidth. Thus a balun is needed which has low insertion losses, low reflection losses and a large bandwidth. Particularly a balun transformer is needed which is small enough to enable practical applications in MMIC. Still further a balun is needed which can be used for microwave frequencies which are low, particularly below about 10 GHz. Still further a balun transformer is needed which is easy and cheap to design, and fabricate, and which is compatible with MMIC technology for frequencies below 10 GHz. Particularly a small size, low loss balun is needed which is compatible with standard silicon MMIC production technology without requiring introduction of additional masks and processing steps. Generally a wideband balun transformer is needed which has a small size, and which can be produced on low resistivity substrates, i.e. substrates with a high conductivity such as for example silicon as used in conventional IC production.
Therefore a multilayer balun signal transformer is provided which comprises a coil system comprising a first coil and a second coil providing at least one balanced signal port at one side of the balun transformer and an unbalanced signal port at another side of the balun transformer, wherein the at least one balanced signal port is provided by a first balanced signal terminal and a second balanced signal terminal formed by the ends of the first coil, the unbalanced (single-ended) signal port being provided by a first unbalanced signal terminal and a second unbalanced signal terminal. The balun is a discrete component and it is formed on a low resistivity, e.g. a semiconductor, substrate layer, and the first and the second coils are formed in, or constitute a first and a second metal layer, wherein between said first and second metal layer and between said second metal and the substrate layer, first and second dielectric layers are disposed. The coil system comprises two (interconnected) discrete coils and at least a portion of one of the coils is disposed in or constitute a metal layer above the metal layer in which at least a portion of the other coil is disposed, or which is constituted by at least a portion of the other coil.
In the following, when referring to coils in a metal layer it may, or generally is meant, that the coils/portions of coils constitute a metal layer/portion (part) of a metal layer.
In a particular implementation the balun transformer is formed directly on the low resistivity substrate layer. Particularly the substrate layer comprises a thin film or bulk semiconductor substrate, e.g. of silicon and the balun transformer is implemented as a MMIC. In a particular implementation each of said first and second coils comprises three or less winding turns. In a most advantageous implementation each of said first and second coils comprises one winding turn each.
In a particular, advantageous implementation, allowing a particularly high input to output coupling and equal impedance of the ports, the first and second coils are arranged symmetrically with respect to the substrate layer. Particularly a first portion of each of said coils are provided in one of said first and second layers whereas the second portions of said first and second coils are provided in the other of said first and second layers whereby via connections are used to interconnect the first and second portions of the respective coils. It is supposed to be well known in the art that a via connection is an aperture formed in a multilayer structure which is plated with conductive material to establish electrical connections at desired points between different layers in the multilayer structure.
Such an implementation is advantageous in that the impedances of the ports will be substantially equal and in addition thereto, the input/output coupling will be even better.
In an alternative embodiment the entire first coil is provided in, or constitute, one of the metal layers whereas the entire second coil is provided in, constitute, the other metal layer. Irrespectively of whether each coil is provided in, constitute, a separate metal layer or if one portion of each coil is provided in one metal layer and the other portion in another metal layer, a capacitor may be integrated in parallel with at least one of the signal terminals, advantageously with at least one of the balanced signal terminals. In alternative implementations a capacitor is integrated in parallel with two or three of the balanced signal terminals and in still other implementations a capacitor is integrated in parallel with each one of the signal terminals. Through introducing capacitors in parallel with the inductors (formed by the signal terminals) the matching between input and output impedances is improved. Such a capacitor, in parallel with the inductor (terminal) will form a resonant tank or a resonant circuit. The impedance of such a circuit may be adjusted by properly selecting inductance and capacitance values. In order to have a transformer having the same input and output impedance, the condition L1=L2=M should be fulfilled, or in other words the inductance of the first coil should be the same as the inductance of the second coil, which should be the same as the mutual inductance between the coils. According to the invention the capacitors may be used to enable a proper control of inherent parasitic capacitances of the coils allowing a substantial size reduction and full on-chip integration in standard silicon technology.
In one advantageous implementation the first and second dielectric layers comprise low loss dielectricas such as e.g. SiO2 or a similar material, e.g. any appropriate bulk or thin film semiconducting material. According to one implementation the first portion of the first coil and the first portion of the second coil are provided in the same first metal layer, their respective first portion constituting the respective signal terminals in an alternative implementation the first portion of the first coil and the first portion of the second coil are provided in the second metal layer, said first portions constituting the respective signal terminals. Particularly the second unbalanced terminal of the second coil is connected to ground, i.e. has zero potential.
In an alternative implementation the first coil is provided in the first layer whereas the second coil is provided in the second layer or vice versa. The potentials at the first and second balanced terminal are particularly equal and opposed, which means that at the midpoint between theme the potential is zero, and said point may be used as a virtual ground, e.g. the second terminal for the unbalanced port, it merely, together with the first unbalanced terminal, forms the unbalanced port. The virtual ground may be connected by a via to the second terminal of the second coil. It is an advantage of such a design that no via connection is required from the first unbalanced terminal to the ground plane which enables a simple design and fabrication. Furthermore, since there is no via connection, the parasitic inductances and capacitances associated therewith will be excluded which will further assist in improving the electrical performance of the balun. Still further, substrate losses will be low since there will be a reduction in induced substrate currents. First, the longitudinal substrate currents induced by the coil strips will almost cancel each other since the currents in the upper and lower, i e. the first and second coils, are equal in magnitude and oppositely directed. However, the canceling of the substrate currents will not be entirely perfect since the distance from the upper coil strip to the substrate will not be the same as the distance from the lower coil strip to the substrate. Partial canceling may also be due to the differences in the thickness and width of the upper and bottom strips. Such partially balanced substrate currents flow in opposite directions and the substrate currents will may be reduced further by choosing the optimum distance between opposing branches of the coils as disclosed in the Swedish patent application xe2x80x9cBalanced inductorxe2x80x9d, application number 9901060-5 filed 990323 by the same applicant and which herewith is incorporated herein by reference.
In one particular implementation, relevant for all embodiments as described above, the/each capacitor comprises a varactor, e.g. a semiconductor varactor, a ferroelectric varactor or a microelectromechanical varactor. Varactors can be used to enable a (better) adjustability of the frequency band. Such adjustable capacitances may also be used to take care of fabrication tolerances etc.
According to the invention it is possible to design the balun and select the values of the capacitor(s) to shift the passband within a frequency range such that a desired passband can be selected.
Through the introduction of capacitors, the bandwidth of the balun will be somewhat reduced as compared to the case when there are no capacitors. However, for most applications, even if there is provided a capacitor for each one of the terminals, the bandwidth will be satisfactory. Particularly it is possible to realize on-chip baluns for frequencies down to at least 5 GHz which particularly have a size of about 0.1 mm2 or even smaller than that.
According to the invention a discrete balun, in which each coil has a low number of winding turns, is provided, particularly only one winding turn for each coil, which will give an input impedance similar to the output impedance in parallel with an inductor. Advantageously there is one capacitor on the input and one on the output to remove the inductive part of the impedance. It is possible to connect such a capacitor only to the in- or the output even if it is particularly advantageous to connect capacitors on both sides. However, to provide a particularly small balun, it is possible to provide a capacitor only on one side, and particularly integrated with only the one of the terminals. This is not possible for distributed baluns. Various implementations of the inventive balun transformer are possible and it can be used for several applications. In one particular embodiment, the balanced terminals are connected to a balanced transmission line, e.g. a coplanar strip line whereas the unbalanced terminals are connected to an unbalanced transmission line, e.g. a microstrip line or a coplanar waveguide.
In a particular implementation the size of the balun is less than about 0.1 mm2.
It is an advantage of the invention that, in addition to other advantages mentioned in the foregoing, a balun what is much smaller than hitherto known baluns can be provided which in addition thereto is a wideband device and which may be produced on high conductivity substrates such as for example silicon used in standardised production of integrated circuits, particularly MMIC.